DD200226A5 - ORGANOMAGNESIUM SOLUTIONS LOW VISCOUSITAET - Google Patents

Abstract

The invention relates to an organomagnesium solution of low viscosity for use in the chemical industry, for example in polymerization processes, for the preparation of polymerization catalysts and others. The object of the invention is to produce low viscosity solutions which are more reactive and in which the solid by-product of polymerization catalysts and the like are obtained. The aim of the invention is to produce low-viscosity solutions which are more reactive and in which the solid by-products readily settle. According to the invention, the novel organomagnesium solutions consist of (a) a hydrocarbon-soluble dialkylmagnesium compound having from 4 to 20 carbon atoms) an organometallic additive selected from the group consisting of R 3 Ga, R 3 deep and In Li, where in the latter R C is deep 1 ... C deep 12-alkyl or C deep 5 ... C deep 7-cycloalkyl. Further, a process for preparing such a solution by in-situ production of the dialkylmagnesium compound via the reaction between metallic magnesium and the appropriate alkyl halide in the presence of both the solvent and the organometallic additive.

Description

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Or ganomagnesium solutions of low viscosity Field of application of the invention

The invention relates to a low viscosity organomagnesium solution and to a process for preparing such solutions. ,

The compounds of the invention are used in the chemical industry in polymerization processes, for the preparation of polymerization catalysts and catalyst intermediates.

Cha rakteristik the known tec hnischen olution

It is known to use organomagnesium compounds for a wide range of chemical reactions. Organomagnesium compounds are used as reagents to reduce ketones, to metalate aromatic compounds and to alkylate metal halides or oxides. As catalysts, the organomagnesium compounds prove their value in the dimerization and polymerization of olefins (see GB-PS Ur. 1 251 177), the polymerization of epoxides (see US Patent No. 3 444 102 ^ and the production of telomers (see US Pat. PS No. 3,742,077) "Organomagnesium compounds are able to perform many of the functions performed by Grignard reagents and, due to differences in electronic and steric factors, exhibit increased reactivity towards certain compounds." For further discussion of organomagnesium reactions Reference is made to U.S. Patent Nos. 3,646,231 and 3,822,219.

Some of the. most useful..Organomagnesium compounds are

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the dialky! magnesium compounds. Although some of them are insoluble in hydrocarbon solvents, it has been found that those compounds having branched chain alkyl groups, cyclic alkyl groups or straight chain groups of five or more carbon atoms are quite soluble. Examples include di-tert-butyl magnesium, di-sec-butyl magnesium, di-n-amyl magnesium, methyl isobutyl magnesium, ethyl isobutyl magnesium, di-n-hexy! Magnesium, dicyclohexyl magnesium, di-n-heptylmagnesiura, etc. In addition, certain Combinations of straight-chain lower alkyl groups also proved to be soluble - n-butylethyl magnesium, n-butylmethylmagnesium, and n-propylmethylmagnesium.

Unfortunately, most of the resulting solutions are highly viscous. This limits the usefulness of the compounds, their viscosity makes them less reactive as reagents and catalysts, and makes their handling and transportability difficult. In addition, the viscosity of the solutions makes it difficult to formulate the compounds in a form free of halides and other undesirable solids herzustellen.-line with the at Glaze.und Selman, Jour nal of Organometallic Chemistry, Vol. 5, "p.477 (1967) and Smith WH, Journa l of Organometallic Ch emistry, VoI 64, p 25 (1974), described Procedure Dialkyl magnesium compounds are advantageously prepared by reaction between metallic magnesium and the appropriate alkyl chloride in the desired hydrocarbon solvent. The by-product of this reaction is the hydrocarbon-insoluble magnesium chloride? Both the magnesium chloride and possibly not in reaction

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gone metallic magnesium, which is often used in excess, remain as a solid, suspended in a viscous liquid substance. The viscosity prevents easy separation of the solution from the pesticides and thus makes a Zentrifugie.r device ο. Ä., Or a long period of time to stop the pesticides required. ,

Object of the invention

The aim of the invention is to provide novel low-viscosity organomagnesium solutions from which impurities are easily separable.

Presentation of the "yes of the invention".

The invention has for its object to reduce the viscosity of organomagnesium solutions by suitable additives.

It has now been discovered that a significant reduction in the viscosity of dialkylmagnesium compounds dissolved in hydrocarbons can be achieved by incorporating organometallic gallium, indium or lithium compounds into the solution. , '

In particular, the invention consists in a low-viscosity liquid solution

•. (a) a hydrocarbon-soluble dialkylmagnesium compound having 4 to 20 carbon atoms per molecule,.

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(B) a solvent from the group of aliphatic, alicyclic and aromatic hydrocarbons having 5 · ν · 20 carbon atoms and

(c) an organometallic additive of the groups R-Ga, RoIn and RLi, in which R is a Cj. .Cj ₂ "" alkyl or a CU ... Cy-cycloalkyl.

The present invention more particularly involves a process for preparing the above-described hydrocarbon solution by reaction between metallic magnesium and the appropriate alkyl halide in the presence of both the desired solvent and the organometallic compound. The reduced viscosity of the resulting solution greatly facilitates this process. Other aspects of the present invention will become apparent from the description below.

The three major components of the present invention are a hydrocarbon soluble dialkylmagnesium compound, a hydrocarbon solvent and an organometallic gallium, indium or lithium compound. To the. Further understanding of this invention, each of these components will now be discussed in its general and specific aspects ·

The dialkylmagnesium compound is one of those compounds having 4 »e.2O carbon atoms per molecule which are known to possess considerable solubility in hydrocarbon solvents. Included here are magnesium alkyls in which at least one of the two groups attached to the magnesium atom consists of a

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branched-chain alkyl group, a cyclic alkyl group or a straight-chain alkyl group having five or more carbon atoms. As long as this condition is met, the two attached groups can be equal to each other or

Examples of dialkylmagnesium compounds corresponding to this description include ifethyl isobutyl magnesium, ethyl isobutylmagnesium, di-n-amylmagnesium, n-butyl-n-hexylmagnesium, ethyl-n-hexyl magnesium, di-n-hexylmagnesium, dicyclohexylmagnesium "Di-n-heptylmagnesium, etc." Also included among the dialkylmagnesium compounds as discussed herein are those compounds in which the two alkyl groups are unbranched, have fewer than five carbon atoms per alkyl group, but differ in length by at least two carbon atoms magnesium and n-propylmethylmagnesium, since these compounds are also known for their solubility in hydrocarbons. "This description indicates that the term" alkyl "in the term" dialkylmagnesium compound "includes the inclusion of straight-chain, light-chain, and hexavalent is intended to cyclic groups, and that the range of 4 »» · 20 carbon atoms whose Ges denotes the atomic number in the two groups on each magnesium atom. "Dialkylmagnesium compounds in the range of 5 ··· 15 carbon atoms are preferred. All indications in this application about ranges of carbon atoms include the upper and lower limits ₀

The concentration of the dialkylmagnesium compound in the solvent is not critical and can vary over a wide range. In many cases, the solubility of the compound is enhanced by incorporation of the organometallic compound. In general, however, viscosity increases with concentration. The preferred concentration therefore varies between about 0.2 and about 50.0% by weight of dialkyl magnesium compound, more preferably between about 1.0 and about 25.0% by weight.

The term "hydrocarbon solvent" is used herein to refer to aliphatic, alicyclic and aromatic hydrocarbons. Examples of aliphatic solvents are n-pentane, isopentane, η-hexane, n-heptane,

n ~ octane, isooctane, pentamethylheptane, gasoline, and other petroleum frits; examples of alicyclic solvents include cyclopentane, cyclohexane, methylcyclohexane, methylcyclopentane, cycloheptane, and cyclooctane. Examples of aromatic solvents are benzene, toluene, xylene, ethylbenzene, tetralin and ζ /. -Methylnaphthalene ,, Preferred solvents contain 5 ··· 20 carbon atoms, more preferred solvents contain 6 ... 15 carbon atoms.

The term "organometallic compound" is used herein to refer to the viscosity-reducing agent, it stands for alkylated gallium, indium or lithium compound of the formulas

R ₃ Ga, R ₃ In or RLi,

in which R corresponds to a Ο,,, CC ₁₂ -alkyl or a C, -, .C -cyclic alkyl. The three groups in the gallium and indium compounds may be the same or different. Preferably, alkenyl groups on the molecule are equal to each other, and R is a CL, Cg alkyl. The term "alkyl" as used in the definition of the organometallic compound includes both straight-chain and branched-chain groups. Examples of organometallic compounds in the context of the present invention are trimethylgallium, triethy gallium, tri-n-propylgallium, triisobutylgallium, n-butyldiethylgallium, n-hexyldimethylgallium tri-n-hexylgallium, trimethylindium, triethylindium, tri-n-butyl indium, tri -'sek-butylindium, n-propyllithium, n-butyllithium, tert-butyllithium, sec-butyllithium, n-amyllithium, n-hexyllithi, etc. Normally hydrocarbon solvent-insoluble organometallic compounds are as useful as normally soluble compounds, - since the dialkylnagnesium compounds exerts a solubilizing effect and thus dissolves insoluble organometallic compounds.

The organometallic compound can be added in its actual form of the solution, but it can also in situ from a

Halide precursors are generated. Typical precursors are GaX, InXo or LiX in which X is chlorine, bromine or iodine; Chlorine and bromine are considered to be most suitable. Chlorine When added to solution, the precursor is subjected to an exchange reaction with the dialkylmagnesium compound in which the halogens are replaced by alkyl groups and magnesium halide precipitates out of the solution. The pesticides are then easily removed, it remains a low-viscosity solution.

The proportion of the organometallic compound in the present composition is not critical and can vary over a wide range, as only a very small proportion is required to effect a marked reduction in solvent viscosity. In general, however, it will be most convenient and inexpensive to use such a proportion which results in a molar ratio of organometallic compound to dialkyl magnesium compound of from about 0.001: 1 to about 0.25: more preferably from about 0.001: 1 to about 0.10 : 1, best from about 0.01: 1 to about 0.05: 1 «

The solution can be readily prepared by physically combining the three founding components. The solubilization can be accelerated by moderately heating and / or stirring the resulting mixture. The resulting solution is clear, the compounds remain in solution even with subsequent Temperaturverminderung.

The dialkyl magnesium compound itself can be prepared by any of various known techniques. One of these techniques is the mercury magnesium exchange method of Cowan and Mosher, Journal of Organics Chemistry , Vol. 27, p. 1 (1962), which describes the subsequent reaction

R ₂ Hg + Mg - ^ R ₂ RIg + Hg, where R is alkyl- ₀ mercury precipitates as lifting

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product from the solution and can be easily removed as such.

Another technique is the dioxanate precipitation method set forth by Schlenk in Reports of the Deutsche Chemischen Gesellschaft , Vol. 64, p. 734 (1931) * The following reaction takes place here:

Ether. , j

2 RMg + C ₄ H ₈ O ₂ > R ₂ Mg + C ₄ H ₈ O ₂ .MgX ₂ ♦

R is alkyl and X is halogen. The precipitated complex is then removed by filtration from the solution; there remains an etherified dialkylmagnesium complex from which the ether must be removed prior to use of the dialkylmagnesium compound in the present composition.

On. the third method is the reaction of an alkyllithium compound with a magnesium halide described in US Pat. No. 3,646,231 ':

MgX ₂ + 2 RLi> R ₂ Mg + 2 LiX

R here stands for alkyl and X for halogen. Again, the precipitated lithium halide must be removed from the solution. Further preparative techniques should not be unknown to the experts.

The preferred technique involves the reaction between metallic magnesium and the appropriate alkyl halide. If an alkylmagnesium compound having two different alkyl groups is desired, the two alkyl halides are used either simultaneously or sequentially.

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applies; alkyl exchange results in a product with mixed alkyl groups. The solvent and the organometallic additive may be added before, during or after the reaction, the precipitated or remaining pesticides are then removed by filtration. The organo-metallic additive greatly simplifies the pesticide disposal step, as it does so beyond the reduction

solution viscosity promotes faster settling of the solids.

Commercially available magnesium turnings or other magnesium turnings may be used in this reaction. However, surface-rich forms of the metal should preferably be used. "Since the surface can be increased by grinding, the use of finely divided magnesium powder with a particle size of about 500 microns and below is the most advantageous variant." This form of metal helps to accelerate the reaction rate and minimize the occurrence of Wurzt coupling reactions »

The alkyl halides used in this reaction may be chlorides, bromides, iodides or combinations thereof. For reasons of economy, chlorides are generally preferred. Usually a small amount of halide is present in the solution of the final product. This can be minimized by chloride use rather than by iodide or bromide use, since the observed amount of soluble halide decreases in the order Iy- Br / -Cl

The molar ratio of the reactants can be varied widely as there is no critical range in terms of reaction rate or product yield. However, normally the starting materials will be present in amounts having a magnesium to alkyl halide mole ratio of about 1 , 0 to about 2.0, preferably from about 1.1 to about 1.3 to ensure. "The molar ratios of over 1.0 inherent magnesium surplus affects in a reduction of Wurtz-Kupplun reactions from ''

The reactivity of the alkyl halides increases with increasing chain length. Thus, methyl and ethyl halides react rather slowly and often require the use of one activating the magnesium

Substance to initiate the reaction "The term" magnesium activating agent "is used herein to refer to heat or any substance which, upon contact with magnesium, has a substantial reaction-accelerating effect." Many such agents are known to those skilled in the art , Typical examples include AlCl, 'AlCl 3 ether complexes, N, IT', brain ethylaniline, molecular iodine, Grignard reagents, and dialkyl magnesium compounds or alkyl halides of longer chain lengths. Since methyl and ethyl halides are used only in conjunction with higher alkyl halides. thus extends a kleine.Menge the latter in order to initiate the reaction Ga, ng "a thermal activation is generally carried out at temperatures between about 125 ° C and about 35O ⁰ C - preferably from about 150 ⁰ C to about 250 ⁰ C. "Achieved" At least 10% by mass of methyl or ethyl halide (based on the mass of the metallic magnesium) must be present during thermal activation »

Once the reaction initiated, it can be performed at lower temperatures or, "in a wide temperature range take place" Most conveniently it will be i.ndes, at temperatures between about 50 C and about 200 ⁰ C - preferably between about 80 ⁰ C and about 15O ⁰ C - to drive _e These temperatures do not wear critical nature, they are mostly undndie possibility of Alkylhalogenidzer-reduction at high temperatures determined by practical considerations such as processes Economic considerations, energy saving »

The reaction pressure is not critical; it can range from atmospheric pressure to elevated pressures of a few atmospheres. " If halides of low molecular weight are used, the reaction should proceed at slightly elevated pressure to keep the alkyl halides in solution. "For larger chain lengths, no increased pressures are required."

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Magnesium alkyls are pyrophoric substances that can spontaneously burst into flames when in contact with air. In order to avoid such an ignition and thus also the oxidation of the magnesium, the reaction must take place in an environment which must not contain more than traces of oxygen. Normally, the reaction is carried out in an inert inert gas atmosphere, for example under nitrogen or argon, when using a highly volatile alkyl halide is rather a Alkylhalogenidgasatmosphere used. Dedweiteren the reaction must take place with extensive exclusion of water, since the system components tend to decompose in the presence of water.

The following embodiments are intended to further illustrate the present invention. They are not intended to define or limit the invention in any way whatsoever.

Execution example

The invention is explained in more detail below with reference to some examples.

This embodiment demonstrates the ability of trimethylgallium to reduce the viscosity of a heptane solution of n-butylethylmagnesium, a hydrocarbon soluble organomagnesium compound. The viscosity reduction is utilized in this embodiment to facilitate the separation of the solution from the solids which remain suspended in the solvent after in- situ preparation of n-butylethyl magnesium.

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.A aerosol compatibility test reactor vessel with 9.0 grams (0.370 gram-atom) of magnesium powder (100 mesh size) was charged and placed in a Ö'lwärmebäd of 160 ⁰ C. The reactor was then purged with ethyl chloride gas and brought to thermal equilibrium at a pressure of 5.9 Newton / cm. Then the oil bath was cooled to 105 ⁰ C, the reactor was commercially available with 201 g

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Heptane solvent (approximately 75 % n-heptane, the remainder predominantly isoheptanes) charged. While stirring, additional ethyl chloride was then fed in over 1.5 l, until a total of 10.1 g (0.158 mol) of added ethyl chloride was reached. The oil bath was now cooled to 80 ⁰ C; While stirring, 13.2 g (0.143 mol) of n-butyl chloride were added for about one hour.

The result was a thick sludge of suspended particulate matter including magnesium chloride and unreacted metallic magnesium. For analysis, a sample of the liquid phase was extracted from the sludge containing 0.10 mass% chloride and 1.28 mass% magnesium , The last mentioned fraction corresponds to an amount of 5 »82% by mass n-Buty! ethylmagnesiumβ

The sludge was now set aside to allow the pesticides to settle at room temperature. Gradual settling was limited to 45 hours. Although the settling process was not yet completed after 45 hours, the level of the pesticide was noted on the bottom of the vessel,

After labeling the level of pesticide at the bottom of the vessel, the vessel was shaken manually to return the mixture to the previous di-slip state. Thereafter, trimethyl gallium was added to the slurry to produce an atomic Mg / Ga ratio of 100; to set the pesticides, the sludge was again set aside. The Lö'sun was significantly less viscous, and after an hour it became apparent that the pesticides had settled to a markedly greater extent than had previously been the Pail after 45 hours of letting without the presence of Trimethj'lgallium. Undoubtedly, the presence of trimethylgallium had significantly reduced the viscosity of the solution and d; with a much faster settling speed allows

we

EXPERIMENTAL EXAMPLE 2

This embodiment illustrates the effectiveness of a number of organic-metal additives in the present invention in reducing the viscosity of n-butylethylmagnesium dissolved in heptane.

To test the various additives, a solution of 9.8 mass $ n-butylethylmagnesium in n-heptane was used "The viscosity measurements were carried out on a modified Ostwald viscometer at 3Q ⁰ C · The results of the measurements are set forth in Table I, from which shows that, in the absence of any additive, the solution had a viscosity of 1343 centipoise and with this value well above all others. Additive-providing solutions,

TABLE I

Effect of additives on the viscosity of 9-8% n-C Hql / IgCpHp. Solution in n-heptane

Mol % additive viscosity at 3O ⁰ C 'relative to (centipoise)

No additive available 1343

Ga (CH ₃ ) ₃ 3.39. 8.8 In (C ₂ H ₅ ) ₃ 3.98 26.0

InCl ₃ 3 ^X

HC ₄ H ₉ Li 4.03 373.0

tC ₄ H ₈ Li 3.68 9.7

The indium trichloride test was a qualitative study, the viscosity reported here is an estimate.

AUSPUHRUITC SAMPLE EXAMPLE 3

This embodiment illustrates the trimethylgallium-induced viscosity reduction of a heptane solution of bi-n-hexylmagnesium.

To prepare the solution, a 37 »9 L reactor was charged with 11.4 kg of heptane, 1.0 kg of magnesium powder (100 mesh) and 0.116 kg of preformed di-n-hexylmagnesium (to initiate the reaction) Reactor was heated to 110 ^° C under a nitrogen blanket; 4.15 kg n-hexyl chloride were over one hour then added slowly · After completion of the reaction, the 'settling of the solids was carried out while the temperature was maintained without stirring at 110 ⁰ C "Now were Peststoffe removed die'viskose liquid was analyzed and showed 2-, 02 'mass% of magnesium - corresponding to 16.2% by mass of di-n-hexylmagnesium - in dissolved state, the viscosity of the solution was 3280 centipoise,

To a subset of the viscous solution was added trimethylgallium in an amount causing an atomic Mg / Ga ratio of 120. From the visual assessment and the manual 'pivoting of the solution was clear _s that its viscosity is reduced immediately to less than 100 centipoise * (this was compared with similar solutions .deren viscosity was measured) "

EXPERIMENTAL EXAMPLE 4

This embodiment illustrates the in-situ formation of tri-n-hexylgallium as a viscosity reducer for a di-n-hexylmagnesium solution in n-heptane.

For this test, a second .Probemenge of described in AHsführungi example 3 solution of di-n-hexylmagnesium in n-heptane was used »The precursor Galliumtrichlorid was here

However, the solution was added in an atomic Mg / Ga ratio of about 6. In this case, with shaking and comparative observation, it was found that the resulting solution had a viscosity approximately equal to that of n-heptane, i.e., "less than 10 centipoise."

Claims (26)

23131 1. Organomagnesium solutions of low viscosity, characterized by ,, that they (a) a hydrocarbon-soluble dialkylmagnesium compound of 4 ··· 20 carbon atoms per molecule, (B) a solvent from the group of aliphatic, alicyclic and aromatic hydrocarbons having 5 · »20 carbon atoms and (c) comprising an organometallic additive, the latter being selected from the group consisting of R., Ga, R-, and Ιη RLi group in which R a C .... C ₁₂ alkyl or a Cc .. .C ~ -Cycloalkyl corresponds. 2. Low-viscosity solution according to item 1, characterized in that the dialkylmagnesium compound contains 5 *. 15 carbon atoms per molecule. 3 * Low-viscosity solution according to item 1, characterized in that the concentration of the dialkylmagnesium compound is about 0.2% to about 50% by mass. 4 A low-viscosity solution according to item 1, characterized in that the concentration of the dialkylmagnesium compound is about 1.0% by mass to about 25.0% by mass. Low viscous solution according to item 1, characterized in that the solvent has been selected from the group consisting of aliphatic, alicyclic and DJO 6 # Medrigviskose solution according to item 1, characterized in that the organometallic additive · was selected from the group consisting of R ~ Ga, IUIn.and RLi, in which R corresponds to a G _1. · .Cg-alkyl. Low viscous solution according to item 1, characterized in that the organometailic additive is generated in situ from the corresponding metallic halide, with Ie1 teres selected from the group consisting of GaX, InX, and LiX, in which case X is selected the group consisting of chlorine, bron and iodine is selected, 8 'Low-viscosity solution according to item 1, characterized in that the organometallic additive is trimethyl] gallium, Low-viscosity solution according to item 1, characterized , that the organometallic additive is generated in situ from gallium trichloride, 10, highly viscous solution according to item 1, characterized in that the organometallic additive is present in a molar ratio of about 0.0 (: 1 to about 0.25: 1) with respect to the dialkylnagnesium compound » 11 "Low-viscosity solution according to item 1, characterized in that the organometallic additive is present in a molar ratio of about 0.0 (: 1 to about 0.10: 1) with respect to the dialkylmagnesium compound." 12 »Low-viscosity solution according to item 1, characterized in that the organometallic additive with respect to the dialkylmagnesium compound in a molar ratio of about 0.0: 1 to about 0.05 'x 1» Process for the preparation of a dialkylmagnesium compound dissolved in a hydrocarbon solvent, characterized in that (a) the metallic magnesium reacts with at least one selected alkyl halide to form a hydrocarbon-soluble dialkylmagnesium compound containing 4 ··· -VT- Carbon atoms per molecule, in the presence or by subsequent addition of a hydrocarbon solvent and an organometallic additive selected from the group consisting of R-Ga, R-In and RLi, wherein. in the latter R a in C. "," C. "Alkyl or a C ₁ - * ·· C ₇ ~ cycloalkyl, and in this way a homogeneous liquid and pests are formed and (b) said homogeneous liquid is separated from said pesticides, wherein both steps are carried out in the substantial absence of both water and oxygen, 14 · Method according to item 13 », characterized in that the" dialkylmagnesium compound 5 " «15 carbon atoms per molecule * contains" 15e method according to item 13 », characterized in that the solvent is selected from the group consisting of aliphatic, alicyclic and aromatic hydrocarbons with β _ββο 15 carbon atoms. 16 »Procedure under point 13? characterized in that the organometallic additive is selected from the group consisting of R ^ Ga, R-In and RLi and wherein R corresponds to a C _o . # C, - ~ alkyl. 17. A method according to item 13, characterized in that the organometallic additive with respect to the dialkyl magnesium compound is present in a molar ratio of about 0.001: 1 to about 0.25: 1. , , 18. The method according to item 13 », characterized in that the organometallic additive with respect to the dialkylmagnesium compound in a Mo! Ratio of about 0.001: 1 to about 0.10: 1, " 19. The method according to item 1, characterized in that the organometallic additive with respect to the Dialkylmagnesiur compound in a Mo! Ratio of about 0.01: 1 to about 0.05: 1 is present. '· 20, method according to item 13 »characterized in that the metallic magnesium is composed of particles of 500 microns in diameter and below. 21 »Process according to item 13», characterized in that the molar ratio of magnesium to alkyl halide eta is from 1.0 to about 2.0 » 22 "Method according to item 13", characterized in that the molar ratio of magnesium to alkyl halide is about 1.1 to about 1.3, 12/22/1981 AP C. 07. P / 231 59 261 18 aromatic hydrocarbons having 6 ... 15 carbon atoms. , 12/22/1981 AP C 07 .F / 231 316 59 261 18 invention claim 23 "Method according to item 13", characterized in that it is alkyl chloride in the alkyl halide. 24. The method according to item 13, characterized in that the organometallic additive is generated iji situ from the corresponding metallic halide, the latter being selected from the group of GaX, InX, and LiX and in turn X from the of chlorine, bromine and iodine existing group comes from 25. The method according to item 13 »characterized in that it hanc in the organometallic additive to trimethylgallium 26. Method according to item 13 », characterized in that the organometallic additive is produced in .situ from gallium trichloride _e . ,